The present invention relates to a nuclear magnetic resonance spectrometer and, more particularly, to a nuclear magnetic resonance spectrometer adapted to obtain visual information about nuclei exhibiting long relaxation times such as .sup.31 P.
In recent years, many studies have been made of instruments which visualize the spin density distribution of .sup.1 H (hydrogen nucleus) on a cross section across a living organism making use of nuclear magnetic resonance. Ordinary nuclear magnetic resonance spectrometers apply a uniform magnetic field to a sample to obtain a resonance signal. In the aforementioned visualizing instruments, a gradient magnetic field having spatially different strengths is superimposed on such a uniform magnetic field that is applied to a subject to be visually observed. A single transmitter/receiver coil which is mounted so as to surround the subject applies an RF magnetic field in the form of a pulse to the subject. After the application, a resonance signal is detected by the coil, and the resultant signal is processed by a computer to obtain the spin density image of an arbitrary cross section across the subject. In this case, the image represents the distribution of the water in the living organism.
Similar measurement of the spin density distribution of other nuclide, such as .sup.31 P, which exists in the living organism may be contemplated as described in "Journal of Magnetic Resonance 38. 343-356 (1980)." However, as well known in the art, the sensitivity with which nuclides other than .sup.1 H are detected is much lower than that with which .sup.1 H is detected. For example, the sensitivity for .sup.31 P is lower than the sensitivity for .sup.1 H by a factor of about 16. To make up for this poor sensitivity for .sup.31 P, the accumulation period of time is required to be increased by a factor of at least 256. Further, the .sup.31 P chemical shift splits each peak into a plurality of lower peaks. This also needs an increase in the number of spectra accumulated. Furthermore, for .sup.31 P the relaxation time is as long as several seconds, during which the next measurement for the accumulation cannot be carried out, thus lenghthening the time taken to make all the measurements. Additionally, the concentration of .sup.31 P in a living organism is lower than that of .sup.1 H. For these reasons, the measurement of .sup.31 P necessitates a time that is one or two thousand times longer than the time required to measure .sup.1 H.